This invention relates to a vapor generator and, more particularly, to a sub-critical or super-critical once-through vapor generator system for converting water to vapor.
In general, a once-through vapor generator operates to circulate a pressurized fluid, usually water, through a vapor generating section and a superheating section to convert the water to vapor. In these arrangements, the water entering the unit makes a single pass through the circuitry and discharges through the superheating section outlet of the unit as superheated vapor for use in driving a turbine, or the like.
These arrangements provide several improvements over conventional drum-type boilers, and, although some problems arose in connection with early versions of the once-through generators, such as excessive thermal loses, mismatching of steam temperature, the requirement for sophisticated controls and additional valving during startup, these problems have been virtually eliminated in later generation systems.
For example, the system disclosed in U.S. Pat. No. 4,099,384 issued on July 11, 1978, and assigned to the assignee of the present invention, includes a plurality of separators disposed in the main flow line between the vapor generating section and the superheating section and adapted to receive fluid flow from the vapor generating section during startup and full load operation of the system. This arrangement enables a quick and efficient startup to be achieved with a minimum of control functions, and without the need for costly valves. Also, the turbines can be smoothly loaded at optimum pressures and temperatures that can be constantly and gradually increased, without the need of boiler division valves or external bypass circuitry for steam dumping. Also, according to this system operations can be continuous at very low loads with a minimum of heat loss to the condenser.
In the latter arrangement, the walls of the furnace section of the generator are formed by a plurality of vertically extending tubes having fins extending outwardly from diametrically opposed portions thereof, with the fins of adjacent tubes being connected together to form a gas-tight structure. During startup the furnace operates at constant pressure and super-critical water is passed through the furnace boundary walls in multiple passes to gradually increase its temperature. This requires the use of headers between the multiple passes to mix out heat unbalances caused by portions of the vertically extending tubes being closer to the burners than others or by the tubes receiving uneven absorption because of local slag coverage, burners being out of service, and other causes. The use of these intermediate headers, in addition to being expensive, makes it undesirable to operate the furnace at variable pressure because of probability of separation of the vapor and liquid phases within the header and uneven distribution to the down-stream circuit. Therefore, this type of arrangement requires a pressure reducing station interposed between the furnace outlet and the separators to reduce the pressure to predetermined values and, in addition, requires a relatively large number of downcomers to connect the various passes formed by the furnace boundary wall circuitry.
In U.S. application Ser. No. 791,830, now U.S. Pat. No. 4,116,168 filed on Apr. 28, 1977 and assigned to the same assignee as the present invention, a vapor generator is disclosed which incorporates the features of the system discussed above and yet eliminates the need for intermediate headers, additional downcomers, and a pressure reducing station. These improvements are achieved at least in part by forming the boundary walls of the furnace section of the vapor generator by a plurality of interconnected tubes a portion of which extend at an acute angle with respect to a horizontal plane. According to a preferred embodiment of this arrangement, the boundary walls defining the upper and lower portions of the furnace section of the vapor generator extend vertically while the tubes in the intermediate furnace portion extend at an acute angle with respect to a horizontal plane. The latter tubes are in fluid flow registry with the tubes in the lower and upper furnace portions, and wrap around the furnace section for at least one revolution.
This use of angularly extending tubes in the intermediate furnace section enables the fluid to average out furnace heat imbalances and be passed through the boundary walls in one complete pass thus eliminating the use of multiple passes and their associated mix headers and downcomers. As a result, the furnace can be operated at variable pressure and the need for a pressure reducing station is eliminated. Also as a result of the angularly extending tubes, a relatively high mass flow rate together with a large tube size is possible compared to a vertical tube arrangement.
However, although the use of the angularly extending tubes has apparent advantages, there is a problem associated with their use. In particular, in a typical vertical tube arrangement, the gravity loads on the furnace walls formed by the tubes results in negligible bending stress in the tubes. However, when the tubes are disposed at an angle, i.e., with their longitudinal axis at an angle to the vertical, a component of gravity load perpendicular to the longitudinal axis is introduced, which causes bending moments in the tube wall and, if not relieved, could result in failure of the wall.